1. Field of the Invention
The present invention is directed t the separation of organic liquid mixtures. The separation of organic liquid mixtures is a common operation in many process industries, especially in the petrochemical industry. When applied to separating close-boiling mixtures, fractional distillation, the conventional technique for separating organic liquid mixtures, is expensive on account of its large energy usage. Frequently, many equilibrium stages, coupled with a large reflux ratio, would be required in order to effect the desired degree of separation. To separate constant boiling binary azeotropic mixtures into their components in substantially pure form, distillation requires adding a third component, and requires the use of more than one column. An example of such a distillation is the process of making absolute ethanol (100%) from a constant boiling azeotropic composition of 95.5% ethanol and 4.5% water. As a third component, benzene is added to the mixture. The resulting distillate produces two liquid layers, one containing practically no water and the other containing most of the water. The use of benzene in such a process not only increases the cost of raw materials, but also entails an additional handling step as well as introducing a potentially environmentally hazardous material into the process.
By contrast, a membrane-based process offers an effective and low-cost alternative to separating close-boiling liquid mixtures by distillation.
In recent years several alternative separation strategies, such as pervaporation, perstraction, and membrane distillation have been suggested. However, the application of any of these techniques depends on finding a suitable membrane for the particular separation problem. For pervaporation --which may be defined as the preferential dissolution in, and permeation through, a membrane of one or more components of a mixture, and removal of the permeate material from the other side of the membrane by evaporation--proceeds through a sorption-diffusion-desorption type mechanism, with the effectiveness of a particular membrane for a given mixture being dependent on the interaction between the membrane and the components present in the mixture.
For the first time, the present invention provides membranes which facilitate a process of separating close-boiling liquids or azeotropic liquid mixtures, as well a novel process of manufacture of such membranes.
2. Description of Related Art
Few publications mention the separation of organic azeotropes using polymeric membranes. In Aptel et al, J. Memb. Sci., 1 (1976), pages 271-287 is described a pervaporation process through poly(tetrafluorethylene) film grafted with N-vinylpyrrolidone to fractionate positive azeotropic liquid systems. Selectivities higher than 10 have reportedly been obtained and daily productions above 100 kg per square meter of membrane have been reported. The pervaporation flux, (.phi.), expressed in kg/hour per meter squared of membrane, was quite low. Other attempts for separating close-boiling, heat sensitive mixtures are set forth in U.S. Pat. No. 4,728,429 which also contains a description of various prior art membrane-forming materials and processes at columns 1-3 of its disclosure. It is quite evident from the description of these various prior art membrane-forming materials and processes of use that the art is searching for a membrane which is not only highly selective but also permits high pervaporation flux so as to facilitate the utilization of the membrane in a commercial process for separation of close-boiling liquids and/or liquid azeotropic mixtures. Other disclosures of membrane compositions and the use of such membranes can be found in the following publications: Cabasso I., Ind. Eng. Chem. Prod. Res. Dev., 22 (1983), pages 313-319; Japanese Patent 58 58 104, issued to Kuraray Co. Ltd. Sin-Etsu Chem. Industry Co. Ltd., on Apr. 6, 1983, and U.S. Pat. Nos. 4,470,859, 4,663,013 and 4,778,596. The disclosures of each of the above-mentioned publications and patents are herein incorporated, by reference, in their entireties.
From the activity in this area, as represented by the foregoing publications and patents, it is evident that a long felt and continuing need for novel membranes is required in order to successfully separate organic liquid mixtures, especially close-boiling liquids and azeotropic liquid mixtures in a commercially feasible process. The membranes of the present invention are formed by a unique process of manufacture.